The present invention relates to a semiconductor device and a method of manufacturing a semiconductor device which can control the lifetime of minority carriers.
Conventional switching elements using silicon include transistors, silicon rectifying (SR) elements, silicon control rectifying (SCR) elements, and the like. An insulating film consisting of SiO.sub.2, low-melting point glass or a resin is used as a protecting film for protecting a silicon substrate having such elements formed therein or a surface of a rectifying junction. Control of minority carriers in such elements is performed by diffusion of a heavy metal such as Au or Pt. According to this heavy metal diffusion method, as shown in FIG. 1A, an Au diffusion source film 4 is formed on a substrate wherein a p.sup.+ -type layer 3 is formed on an n.sup.+ -type layer 1 through an n.sup.- -type layer 2. Annealing is performed to diffuse the heavy metal in the substrate, as shown in FIG. 1B. Thus, a deep level is formed in the semiconductor bandgap such that it serves as the recombination center for the minority carriers. Thus, the lifetime of the minority carriers is decreased to control the switching characteristics of the elements. However, with the heavy metal diffusion method, the thermal diffusion and hence the controllability depends upon the annealing temperature. Furthermore, this method has a low design allowance with respect to temperature and can therefore limit the yield of semiconductor devices.
A method for irradiating the .gamma.-rays or with an electron beam controls the lifetime of minority carriers in the following manner. For example, as shown in FIG. 2A, an emitter region 8, a guard ring region 9, and a channel-cut region 10 are formed in a base region 7 and other surrounding portions of an n.sup.- -type layer 6 on an n.sup.+ -type layer 5. A thermal oxide film 11, an Al electrode 12 connected to the emitter region 8, and a CVD (Chemical Vapor Deposition) PSG film 13 covering them are formed on the surface of the structure to provide a semiconductor substrate. Them, as shown in FIG. 2B, the semiconductor substrate is irradiated with a high-energy electron beam to intentionally form crystal defects which will serve as recombination centers for minority carriers. According to this method, the lifetime of the minority carriers can be controlled with high precision by intentionally introducing crystal defects. However, when such crystal defects are formed, defects are also formed in an insulating film and positive charges are generated in the insulating film. Generation of these positive charges results in a decrease in junction breakdown voltage. This phenomenon is particularly notable in the case of high breakdown-voltage elements. The degree of decrease in the junction breakdown voltage depends upon the dose of radiation. When the radiation dose increases, the decrease in the breakdown voltage also increases.